The NASA astronaut knocks loudly three times on a what appears to be a nondescript door, and calls cheerfully: "You ready to come out?"

The reply is inaudible, but beneath his mask, he appears to be grinning as he yanks the door open -- and four scientists who have spent a year away from all other human contact, simulating a mission to Mars, spill out to cheers and applause.

Anca Selariu, Ross Brockwell, Nathan Jones and team leader Kelly Haston have spent the past 378 days sealed inside the "Martian" habitat in Houston, Texas, part of NASA's research into what it will take to put humans on the Red Planet. 

They have been growing vegetables, conducting "Marswalks," and operating under what NASA terms "additional stressors" -- such as communication delays with "Earth", including their families; isolation and confinement. 

It's the kind of experience that would make anyone who lived through pandemic lockdowns shudder -- but all four were beaming as they reemerged Saturday, their hair slightly more unruly and their emotion apparent. 

"Hello. It's actually so wonderful just to be able to say hello to you," Haston, a biologist, said with a laugh.

"I really hope I don't cry standing up here in front of all of you," Jones, an emergency room doctor, said as he took to the microphone -- and nearly doing just that several moments later as he spotted his wife in the crowd. 

The habitat, dubbed Mars Dune Alpha, is a 3D printed 1,700 square-foot (160 square-meter) facility, complete with bedrooms, a gym, common areas, and a vertical farm to grow food.

An outdoor area, separated by an airlock, is filled with red sand and is where the team donned suits to conduct their "Marswalks", though it is still covered rather than being open air.

"They have spent more than a year in this habitat conducting crucial science, most of it nutrition-based and how that impacts their performace ... as we prepare to send people on to the Red Planet," Steve Koerner, deputy director at NASA's Johnson Space Center, told the crowd.

"I'm very appreciative."

This mission is the first of a series of three planned by NASA, grouped under the title CHAPEA -- Crew Health and Performance Exploration Analog.

A year-long mission simulating life on Mars took place in 2015-2016 in a habitat in Hawaii, and although NASA participated in it, it was not at the helm.

Under its Artemis program, America plans to send humans back to the Moon in order to learn how to live there long-term to help prepare a trip to Mars, sometime towards the end of the 2030s.